In a wireless communication system, if a user equipment is initially powered on, the user equipment selects an initial cell in an idle mode. The idle mode means a state for receiving control information of a cell suitable to receive a service when the user equipment is initially powered on, and a standby state after communication. Basically, the user equipment selects the cell to register with a network so that it may receive a service from a base station. In this case, if strength or quality of a signal between the user equipment and the base station is deteriorated due to mobility of the user equipment, the user equipment performs re-selection of another cell to maintain transmission quality of data. Hereinafter, a feature of a physical signal related to signal strength or a signal to noise/interference ratio will be referred to as a signal feature.
FIG. 1 is a diagram illustrating an operation of a user equipment in an idle mode when the user equipment is initially powered on. If the user equipment is powered on, the user equipment automatically or non-automatically selects a public land mobile network (PLMN), which it desires to receive a service, and a radio access technology (hereinafter, referred to as “RAT”) for communication (S400). The PLMN and RAT information may be selected by a user of the user equipment. Alternatively, the PLMN and RAT information previously stored in a universal subscriber identity module (USIM) may be used. Afterwards, the user equipment performs an initial cell selection procedure for selecting a cell having the largest value among cells having signal strength or quality between the user equipment and the base station, wherein the signal strength or quality is measured to be greater than a reference value (S410). The reference value means a value defined in the system to ensure quality of a physical signal during data transmission and reception. Accordingly, the reference value may depend on the applied RAT.
Afterwards, the user equipment receives system information (SI) periodically transmitted from the base station. The system information includes basic and indispensable information required for the user equipment to access the network. Also, the system information can include neighbor cell list (NCL) information which is information related to cells adjacent to a serving cell. Accordingly, the user equipment should receive system information before accessing the base station and always have the latest system information. If the user equipment is initially powered on, the user equipment selects a cell in an idle mode to receive the system information.
A detailed method for selecting a cell in a universal mobile telecommunication system (hereinafter, referred to as “UMTS”), which is a three-generation mobile communication system, will be described below. If the user equipment is initially powered on, the user equipment selects PLMN and RAT for radio communication and selects a cell having the strongest signal characteristics value among neighbor cells which satisfy the condition of Equation 1 through a signal measurement procedure with the base station in all frequency bands that can be detected by the user equipment during an initial cell selection procedure corresponding to step S410 of FIG. 1. In the existing WCDMA UMTS, a common pilot channel received signal code power (CPICH RSCP) corresponding to an average power after dispreading and combining of common pilot channels, a chip energy to noise ratio of a common pilot channel (CPICH Ec/N0), and carrier received signal strength indicator (carrier RSSI) are used as signal measurement target values.Cell selection condition: Srxlev>0 AND Squal>0  [Equation 1]wherein,Squat=Qrxlevmeas−Qrxlevmin−Pcompensation
Meaning of each parameter is defined in Table 1.
TABLE 1ParameterDescriptionSqualCell Selection quality value (dB)SrxlevCell Selection RX level value (dB)QqualmeasMeasured cell quality value expressed in CPICHEc/N0 (dB)QrxlevmeasMeasured cell CPICH RSCP RX level value (dBm).QqualminMinimum required quality level in the cell (dB)QrxlevminMinimum required RX level in the cell (dBm)PcompensationRF dependent value (dB)
As apparent from Equation 1 and Table 1, the user equipment selects a cell of which signal strength and quality are greater than specific values (strength: Qrxlevmin+Pcompensation, quality: Qqualmin) defined in the system. In this case, the values Qrxlevmin, Qqualmin, and Pcompensation are informed from the base station to the user equipment using system information. The user equipment is standby in an idle mode to request a network to provide a service (for example, originating call) or receive a service (for example, terminating call) from the network.
Afterwards, the user equipment registers its information such as international mobile subscriber identity (IMSI) with the network to receive a service (for example, paging) from the network (S420). Although the user equipment which is in an idle mode can receive control information such as system information from the cell, the user equipment is not in a radio resource control (RRC) connected state with UTRAN. Accordingly, since the network cannot identify exact information of the user equipment, the network uses IMSI used on a non-access stratum (NAS). The user equipment does not always register its information with the access network whenever selecting a cell. The user equipment registers its information with the network if network information which the user equipment knows is different from network information (for example, tracking area identity; TAI) included in system information.
If a strength or quality value of a signal measured from a base station which provides a service to the user equipment is lower than a value measured from a base station of a neighbor cell, the user equipment selects one of another neighbor cells which provide a better signal feature than that of the cell of the base station which the user equipment accesses (S430). This procedure will be referred to as a cell re-selection to be distinguished from the initial cell selection of step S410. The user equipment which is in an idle mode repeats the cell re-selection procedure of reselecting a cell having a better signal feature through signal measurement of a neighbor cell of a cell to which a service is currently provided. At this time, to prevent a cell from being frequently reselected in accordance with variation of the signal feature, there are provided temporal restrictions. Alternatively, since the user equipment does not need to perform cell re-selection if a signal characteristics value of a serving cell is greater than a reference value, the user equipment may not perform measurement.
FIG. 2 is a diagram illustrating a method of performing cell re-selection in a user equipment of an idle mode in the existing WCDMA UMTS. The reason why that the user equipment reselects a cell having a better signal feature is to improve service quality by minimizing error occurring during data transmission and reception between the user equipment and the base station. Accordingly, if signal strength or quality between the user equipment and the base station which is currently connected with the user equipment is deteriorated as the user equipment moves, it is necessary to reselect a cell having a better signal feature. However, if the user equipment reselects a cell, the user equipment needs additional operation such as reading-out operation of system information and registration operation with the network to receive a service in the selected cell. A problem occurs in that it is difficult for the user equipment to request a service (for example, RRC connection request) or receive a service (for example, paging) from the base station while the user equipment is performing the additional operation. Namely, delay of a service occurs.
Accordingly, if a signal feature is deteriorated, the user equipment reselects a cell having a better signal feature within a short time and at the same time prevents cell re-selection from being frequently performed to minimize a service delay that may occur. To this end, the user equipment sets minimum restriction time (Treselection) for cell re-selection to prevent a cell from being frequently selected. Also, a scaling factor is applied to the restriction time in accordance with mobility of the user equipment. Namely, if a moving speed of the user equipment is fast, the existing restriction time becomes short in response to fast cell change so that the user equipment quickly reselects a cell.
In more detail with reference to FIG. 2, the user equipment which is in an idle mode searches whether a signal characteristics value Rs of a serving cell and a signal characteristics value Rn with neighbor cells satisfy a condition defined in step S510. In other words, among cells which satisfy the condition of Equation 1, if the condition of Rn>Rs is satisfied for the restriction time Treselection* updated for cell re-selection, the user equipment selects a cell having the largest feature value through a ranking procedure which compares signal strength and quality among cells corresponding to Rn (S520). Namely, the user equipment selects a specific cell having the best signal feature among cells having a better signal feature than that of the serving cell. Rs and Rn are respectively determined through calculation procedures of Equation 2 and Equation 3.Rs=Qmeas,s+Qhysts+Qoffmbms  [Equation 2]Rn=Qmeas,n+Qoffsets,n+Qoffmbms  [Equation 3]
In Equation 2 and Equation 3, Qmeas,s means CPICH Ec/N0 value measured for a cell where the user equipment is currently receiving a service, and Qmeas,n means CPICH Ec/N0 value measured for neighbor cells of the user equipment. Qhysts is used so that the user equipment gives a weight value for the serving cell, Qoffsets,n is used so that the user equipment gives a bias between a cell where the user equipment is accessing and a cell where the user equipment desires to access, or Qoffmbms is used so that the user equipment gives a weight value for a cell which supports MBMS service.
The restriction time Treselection value is used to give time restriction prescribed to satisfy the condition (S510) of cell selection for a predetermined time or greater to prevent the user equipment from repeatedly selecting a specific cell. The Treselection value included in the system information and transmitted to the user equipment is corrected in accordance with the moving speed of the user equipment during the procedure of FIG. 3, so that the updated restriction time Treselection* is determined.
Hereinafter, the initial restriction time transmitted from the base station to the user equipment through the system information will be referred to as “Treselection”, and the restriction time updated by applying a scaling factor determined in accordance with mobility of the user equipment to the initial restriction time through cell re-selection will be referred to as “Treselection*”. The updated restriction time may be equal to the initial restriction time, or may be decreased or increased from the initial restriction time.
FIG. 3 is a diagram illustrating a method of determining mobility, i.e., moving speed of a user equipment for cell re-selection in the existing WCDMA UMTS.
After the user equipment reselects a cell (S600), the user equipment determines its current speed (S610). The moving speed of the user equipment is initially in an idle mode but is changed as the user equipment moves. The user equipment which is in an idle mode determines whether its moving speed is to be changed to fast speed in step S630. If the number of times of cells selected for a specific time TCRmax exceeds a predetermined value NCR, the user equipment determines that its moving speed is fast (S640). If the number of times of cell reselection does not satisfy the condition NCR the user equipment determines that its moving speed is slow (S650). The user equipment continues to determine its moving speed and repeats the above procedure if it determines that its moving speed is still slow. In order to continue to determine that the moving speed of the user equipment is fast, the user equipment determines whether the condition of fast speed is satisfied for a predetermined time TCRmaxHyst (S620) even after determining that its moving speed is fast in step S610. If the condition of fast speed is satisfied, the user equipment maintains fast speed. If not so, the user equipment determines that its moving speed is slow (S650).
If the user equipment finally determines that its moving speed is fast (S640), the user equipment multiplies the value of the restriction time Treselection by a value between 0 and 1 received from the system information so as to update the restriction time which reflects the moving speed of the user equipment during cell re-selection. Namely, according to the related art, the user equipment determines that its moving speed is fast if a cell is frequently changed with another one, whereby the restriction time according to cell re-selection is reduced. As a result, the user equipment can more frequently perform cell re-selection and receive a service from a cell having a better signal feature.
Parameter values such as TCRmax, NCR, TCRmaxHyst, Treselection, and scaling factor are included in the system information in step S410 of FIG. 1 and then transmitted to the user equipment. Table 2 illustrates an example of a range of the above values that can be transmitted from the base station in the UMTS according to the related art.
TABLE 2ParameterValue typeTCRmaxEnumerated (not used, 30, 60, 120, 180, 240),second unitNCRInteger (1 . . . 16)TCRmaxHystEnumerated (not used, 10, 20, 30, 40, 50, 60, 70),second unitTreselectionInteger (0 . . . 31), second unitScaling Factor forReal (0 . . . 1 by step of 0.1)Treselection
As described above, in the method of determining a scaling factor in accordance with the related art, the moving speed of the user equipment is detected using the number of reselection times of cell for a specific time. However, such a related art has problems as follows. First of all, a problem occurs in that the related art does not consider an arrangement structure of a cell. Also, cell re-selection time becomes slow even though a change of signal feature is lowered to a specific value or less, or ping-pong situation may occur, in which specific several cells are repeatedly reselected.
FIG. 4 is a diagram illustrating an example of a procedure of reselecting a cell according to movement of a user equipment in the WCDMA UMTS.
The user equipment passes through a cell C, a cell B, and a cell D for an ongoing procedure from a cell A to a destination zone (zone X of FIG. 4). In this case, since the user equipment passes through several cells for a short time, it is determined that the moving speed of the user equipment is fast. If a scaling factor (for example, 0.5) is applied to the original restriction time Treselection (for example, 10 seconds), the updated restriction time becomes shorter than the original restriction time by half. The user equipment located in the destination zone (zone X) selects cell B, cell D or cell C as a candidate of a cell that can be reselected. Since the user equipment is located at the boundary of several cells and a measurement value of a radio wave measured by the user equipment has a fluctuation, the user equipment repeats re-selection of several cells even though it moves a little. Also, the user equipment more frequently performs cell re-selection due to the short restriction time. For this reason, a problem occurs in that service quality of the user equipment is deteriorated.